1. Field of the Invention
The present invention relates to electrical machines, and more particularly to rotors having wound-field and permanent magnet excitation.
2. Description of the Related Art
The demand for electrical power in automobiles is continually increasing. At the same time, space available for packaging electrical an electrical generator in an automobile is generally decreasing. Also, demands for higher efficiency and higher electrical output at idle are increasing.
In an attempt to meet all of these requirements, several known designs employ a "hybrid" alternator design in which a traditional wound-field rotor is supplemented with permanent magnets. One such design is disclosed in U.S. Pat. No. 4,959,577, issued to Radomski (hereinafter "the '577 patent").
In the '577 patent, permanent magnets are disposed between the fingers of a "claw pole", or Lundell, rotor. The magnets are magnetized in a tangential direction with respect to the rotor's axis of rotation.
Although the design of the '577 patent can provide advantages over standard Lundell rotors, the tangential magnetization of the permanent magnets may not make optimal use of the expensive permanent magnets. That is, pure tangential magnetization may miss opportunities to have some direct linking of permanent magnet flux with the stator windings. Such linking may provide benefits in the form of higher electrical output and higher power density (i.e., power output per unit volume of the alternator) of the alternator.
Therefore, a design which can take better advantage of permanent magnets used in a "hybrid" rotor can provide advantages over the prior art.